The formation of an electrical resistor by doping a zone of monocrystalline semiconductor material, such as, for example, silicon, differs depending on whether the material is initially doped or non-doped. Co-pending, commonly assigned U.S. Pat. application, Ser. No. 540,142 is a related application, and is a counterpart of issued European patent application No. 107,556.
The non-doped monocrystalline material comprises the substrate on which the integrated circuits are formed. A resistor is made in a zone of the substrate by doping it with electrically active ions that are capable of defining a type of conductivity, n or p, in the material. Similarly, electrically inactive ions, or atoms, are not capable of defining a type of conductivity in a semiconductor material. For silicon, the active ions are for example arsenic, phosphorus, or boron. The resistance depends on the concentration (density) of the active ions in the zone in accordance with a complex curve, which makes it difficult to adjust the resistance in a desired manner.
Furthermore, the substrate always undergoes at least one annealing operation subsequent to the formation of the resistor. Experience shows that annea1ing causes the active ions to diffuse in a practical1y anisotropic manner from the zone to its surrounding portions. The density of the ions, and consequently the resistance, thus undergo modification due to annealing. The magnitude of the change depends in particular on the temperature of annealing, on the initial density of the ions, and on the density gradient at the edge of the resistive zone. Since the peripheral portions of the resistive zone are not doped, the gradient is always relatively high. Furthermore, taking into account the many parameters involved, their own very variable effects and their complex influence on one another, it is impossible to adjust the resistance to the desired value within meaningful limits. Additionally, the diffusion notably increases the surface area of the initially doped zone and thus hinders larger-scale integration of the components of the substrate. In practice, this method of resistor formation does not provide any control over the resistance and so it is not used except in cases where the sole constraint is to limit the resistance.
The resistors found in the monocrystalline substrate of an integrated circuit are ordinarily resistive zones contained in the regions that have already been doped. Taking into account the relatively slight resistivity of an initially doped region, the method of producing a resistive zone in this region differs depending on whether its resistance is less than or greater than that of the doped region.
To obtain a smaller resistor, the resistive zone is doped strongly. Nevertheless, such a resistor must extend between two contacts so it can be inserted into an electrical circuit. In that case, the resistance of the region comes to be parallel to the intrinsic resistance of the resistive zone. The effective resistance of the zone then differs from its intrinsic resistance, as a function of the ratio between the intrinsic resistance of the zone and the resistance of the neighboring region of the substrate. To lessen the influence of the substrate resistance, the resistive zone is given a low resistance with respect to that of the substrate. Nevertheless, the strong concentrations of the dopants dictated by such a zone, along with the existence of strong density gradients on the edge of the zone, thus contribute to extensive diffusion of the ions from the initially doped zone to the surrounding region. This diffusion prevents any control over the method, even though such control is necessary, to adjust the low resistances correctly. One remedy is to change the type of conductivity of the ions between the zone and the region, in such as way as to insulate them with a p-n junction. However, the junction imposes relative voltage values, and if the nature of the ions is changed this still does not prevent their diffusion.
The production of a zone of higher resistance in an initially doped region is illustrated in U.S. Pat. No. 4,432,008. The resistive zone is placed on the surface of the region so that it is directly in contact with a conductor of the mesh network of the integrated circuit. In this manner, the resistive zone is located in series with the conductor and the conductive region. This accordingly necessitates a highly doped region within the monocrystal, such as the drain region of a field effect transistor, in the example chosen here. Yet the advantage is to have only one contact with the conductor, and that it is thus possible to reduce the surface area of the resistive zone around this sole contact.
To increase the resistance of the zone in the drain region of a transistor, it is known to introduce inactive dopants into the zone, which for silicon are gold, silver, zinc and copper. However, in practice it is found that all the inactive ions diffuse in the same manner as the active ions during an annealing operation. Further, to notably increase the resistivity of the zone relative to that of the region, a large quantity of inactive ions has to be introduced. The edge of the resistive zone will thus be the site of a marked density gradient. By way of example, for a silicon zone doped with gold ions having a coefficient of diffusion D of 10.sup.-10 cm.sup.2 /sec, and having undergone annealing typically performed at a temperature of approximately 1000.degree. C. for one-half hour, experience shows that the gold ions have diffused to a mean distance of approximately 4 micrometers (4 .mu.m). Thus one finds the same disadvantages as those described above for the method of forming a resistive zone in the non-doped substrate. On the one hand, such diffusion prevents any effective, reliable adjustment of the resistance of the resistive zone. On the other, the diffusion considerably increases the dimensions of the initial resistive zone, and thus hinders large-scale integration of the components in the substrate. Furthermore, inactive ions emitted by the resistive zone and diffused into the neighboring region can contaminate functionally active sectors of the substrate, such as the sector located under the oxide film gate electrode of the field effect transistor, which includes the resistive zone in its drain as in the case of the above-cited U.S. patent. In that case, contamination is considered very deleterious to the transistor characteristics.
In view of the problems encountered in forming a resistor by introducing the inactive ions mentioned above, the aforenoted U.S. patent proposes increasing the resistance by creating defects in the crystalline lattice in the resistive zone. Unfortunately, experience shows that annealing does not allow any but a very small proportion of the initial defects to remain. Still, it is notable that this patent does not mention any examples of the resistances the described method is capable of creating.
Because of all the problems encountered in forming resistive zones in the monocrystalline substrate of an integrated circuit, they are currently inserted into the multilayer mesh network of components of the integrated circuit when the circuit includes a conductor layer made of a polycrystalline semiconductor material, typically silicon. For example, this kind of network is typically found in integrated circuits that include MOS-FET transistors (metal-oxide semiconductor field effect transistors), the gate electrodes of which are made of a polycrystalline semiconductor material. The step of producing these gate electrodes is taken advantage of to form the first level of the mesh network of components of the integrated circuit. The production of this first integrated level includes the deposition of a film of polycrystalline semiconductor material of uniform thickness, etching this film to etch in the connection elements there, and implanting a strong dose of active ions to make the connection elements conductive.
The formation of resistive zones in the polycrystalline connection elements is typically done by a standard method such as that described for example in the article by Ohzone et al., appearing in the journal "The Transaction of the IECE of Japan", Vol. E 63, Number 4, Apr. 1980, pages 267-274. According to this earlier method, an initial implantation of active ions in a small dose is made in a1l the non-doped polycrystalline semiconductor material. The dose is calculated to lower the resistivity of the non-doped material until reaching the value desired, to make resistors in the material. These resistors are thus defined within zones by masking of the initially doped material. Then a second ion implantation is done with a strong dose, to decrease the electrical resistance of the non-masked material, comprising the conductive portions of the film, considerably. Once the ion bombardment has altered the structure of the material, the material is annealed in order to restore the initial structure.
Nevertheless, even in a polycrystalline material, annealing sti11 causes diffusion of the dopants in a practically anisotropic manner, similar to that of the dopants in monocrystalline material. In other words, the diffusion is dependent on the same parameters, such as the density gradient of the dopants, the nature of the dopants, and the annealing temperature. It also depends on the granulometry of the polycrystalline material, which promotes diffusion and makes it greater than that in the monocrystalline material. The resistive zones are thus encroached upon by the ions that are present in high dosage in the neighboring conductive regions, to the point that a zone having a small surface area becomes conductive and loses its role as a resistor, and that in a zone having a large surface area, only a central region will continue to have the desired resistivity. Diffusion thus hinders large-scale integration of the resistors in the polycrystalline material. At present, the desired resistive zones are squares on the order of 2 .mu.m on a side, which are absolutely impossible to obtain using the earlier method. On the other hand, taking into account the many parameters associated with diffusion, their own effect in accordance with complicated functions and their complex influence on one another, the method does not enable mastery in adjusting the desired resistance of a resistive zone within meaningful limits. For example, experience shows that the resistance of polycrystalline silicon varies by a factor of from 10.sup.4 to 10.sup.6, for an ion dose that varies by only a factor of 10.
The wide variation in resistance obtained by this method means that at present it cannot be used except for producing very high resistance, on the order of a gigohm or more. In such resistance ranges, variations on the order of a megohm are relatively inconsequential. Furthermore, such values correspond to part of the slight downward slope of the resistivity curve as a function of the ion doping. The integrated circuits that are associated with this earlier method of resistor production are in particular the static RAMs (random access memories) of the MOS type.
The above-cited U.S. Pat. No. 4,432,008 discloses the formation of a resistor in a zone of polycrystalline semiconductor material by uniformly doping the material with active ions and implanting inactive ions in this zone that are identical to those discussed above with respect to the monocrystalline substrate. However, for the same reasons as above, experience shows that the proposed dopants still diffuse into the polycrystalline material, because of its granular structure. These dopants are thus unable to provide sufficient control for meaningful adjustment of the desired resistance of the resistive zones. On the other hand, the proposed combination of inactive ions with the formation of defects in the polycrystalline material proves by experimentation to be without any substantial effect, because the annealing process virtually restores the initial granulometry of the polycrystalline material. The resistivity values of the resistive zones are thus quite close to the resistivity of the material.
An efficacious solution has been furnished by the assignee of the present application in its the aforementioned European patent application No. 107,556. In brief, a resistor is formed in a zone of polycrystalline material by doping with a strong dose of active ions and by implanting ions of the family of rare gases in a quantity and with an energy sufficient to attain the desired resistivity. With this method, resistors having a very small surface area can be reliably and precisely obtained, within a range on the order of from 250 ohms to approximately 25 kilohms.
This invention was also conceived with the idea of eliminating the density gradient of the electrically active ions that previously existed between the zones and the neighboring conductive portions. This elimination is effected by a uniform doping with a strong dose of active ions in all of the polycrystalline material. In this context, it is estimated that the action of the rare gases in a resistive zone was that of the inactive ions mentioned in the above-cited U. S. patent, and that their improved efficacy with respect to other inactive ions was due to better synergy with the active ions introduced in a strong dose into the resistive zones in order to eliminate the ion density gradient. Because of this elimination, the method is limited to producing low resistance values, because of the strong dose of active ions. For example, experience using argon under the conditions described in the European patent show that the method is limited to making resistors that are precisely adjusted within a range of on the order of 250 ohms to 25 kilohms. In other words, while the standard method cannot be used except for very high resistance, the improved method cannot be used except for very low resistance, and cannot cover the range of resistances in between. Moreover, if it were possible to make intermediate resistors precisely and in a small size, it would be possible to realize new types of integrated circuits responsive to user demands.
In summary, technology is at present faced with the problem of producing a resistor in a zone of monocrystalline or polycrystalline material, whether doped or not. Only the formation of low resistance in a strongly doped polycrystalline semiconductor material has been mastered. This problem is substantially due to the diffusion of the resistive doping ions during annealing. The resultant disadvantages are mainly the lack of control the methods provide in adjusting a resistor to the value desired within narrow tolerances, and the necessity of working with resistors of large dimension, which is incompatible with the very large-scale integration that is desired. An additional problem, in certain cases, is the contamination of the resistive doping ions in functionally sensitive sectors of the substrate, and the alteration of the characteristics of the contaminated components.